Selenium (Se) is incorporated into selenoproteins as selenocysteine to impart anti-cancer, antioxidant, and anti-inflammatory functions. Based on our ongoing studies, under marginally high Se status, immune cells shunt the arachidonic acid metabolism towards the production of cyclooxygenase (COX)-derived anti-inflammatory cyclopentenone prostaglandin (CyPG) 15d- PGJ2 rather than pro-inflammatory PGE2 and thromboxane A2. Many studies have shown that high Se inhibits the initiation, promotion, and progression in rodent models of breast, colon, and prostate cancer, where apoptosis of bulk cancer cells was enhanced. However, studies are yet to be done to understand the effect of Se on cancer stem cells (CSC), which occupy the apex of the developmental hierarchy with properties of self-renewal, multipotentiality, and strong proliferative capacity. Existing therapie to treat cancer do not eradicate the CSCs and thus, aid in the relapse of many cancers. To fill this pervasive gap, we propose to examine the effect of dietary Se on the proliferation of leukemia stem cells (LSC) in the following three models: 1) transplantation of Sca1+kit+GFP+LSCs from C57BL/6 mice transplanted with hematopoietic stem cells (HSC) that express the fusion oncoproteins BCR-ABL or MLL-AF9 as models of chronic myelogenous leukemia (CML) and AML, respectively, and 2) M34+Sca1+kit+ LSCs from Balb/c mice infected with Friend leukemia retrovirus-polycythemia, a model of acute myelogenous leukemia (AML). Based on the compelling preliminary data that COX inhibitors block the effect of Se-dependent ablation of leukemia in-vivo, we hypothesize that supraphysiological levels of dietary Se increases the production of endogenous CyPGs to specifically target LSCs to apoptosis by activating the p53 pathway. The hypothesis will be tested in three Specific Aims: 1) To examine if high Se supplementation causes the ablation of LSCs; 2) To examine the role of COX pathway metabolites in the selective targeting of LSCs in Se-supplemented mice, and 3) To delineate the mechanism of apoptosis of LSCs by 15d-PGJ2 in Se supplemented mice. Given the epidemiological evidence of lowered serum Se levels in leukemia patients as well as higher incidences of adult leukemia in individuals consuming non-aspirin analgesics, our long- term goal is to understand if Se supplementation of leukemia patients would be beneficial.